**Question:** Arrange the cyclins and CDKs in cell cycle from G1 to S checkpoint.

A) CDK6/Cyclin E; B) CDK4/Cyclin D; C) CDK1/Cyclin B; D) CDK2/Cyclin A

**Core Concept:**

The cell cycle is a series of events that lead to cell division and is regulated by a group of proteins called cyclins and cyclin-dependent kinases (CDKs). These proteins play a crucial role in controlling the progression of the cell cycle through different phases. Cyclins are proteins that bind to CDKs and activate their kinase activity, which in turn phosphorylate other proteins involved in the cell cycle regulation. The cell cycle is divided into G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis) phases. Each phase has specific cyclins and CDKs involved in regulating the progression through the cell cycle.

**Why the Correct Answer is Right:**

In the correct answer, we have the cyclins and CDKs involved in regulating the progression from G1 to S checkpoint of the cell cycle:

A) CDK6 and Cyclin E: CDK6 is activated by Cyclin E, which is produced during the late G1 phase. This complex regulates the progression from G1 to S phase by phosphorylating retinoblastoma protein (Rb), leading to its dephosphorylation and release of E2F transcription factors, which promote the expression of genes needed for DNA synthesis.

B) CDK4 and Cyclin D: Cyclin D is produced during the late G1 phase, and its binding to CDK4 activates the kinase activity of CDK4. This complex phosphorylates and inactivates the retinoblastoma protein (Rb), allowing E2F activation and progression from G1 to S phase.

C) CDK1 and Cyclin B: Cyclin B binds to CDK1 (Cdc2) during the late G2 phase, forming the mitotic cyclin-dependent kinase (M-phase-promoting factor, MPF). This complex promotes the progression from G2 to M phase by phosphorylating and inactivating the anaphase-promoting complex (APC) and the cyclin B destruction box (Cds1/Cds2) complex.

D) CDK2 and Cyclin A: CDK2 and Cyclin A are involved in the early S phase, where CDK2 is activated by Cyclin A. This complex phosphorylates and activates the retinoblastoma protein (Rb), allowing E2F activation and progression from S phase to G2 phase.

**Why Each Wrong Option is Incorrect:**

A) CDK6-Cyclin D: Although this complex plays a role in the progression from G1 to S phase, the correct answer involves CDK4-Cyclin D, which is activated during late G1 phase.

B) CDK1-Cyclin B: This complex is involved in the progression from G2 to M phase, not from G1 to S phase, as represented by CDK2-Cyclin A.